The polymerization of lactams in molds has been used and studied for decades. The latest efforts to improve the technique to polymerize lactams have been focused in modifying the catalytic system (formed by the activator and initiator) in order to carry out the polymerization in a controlled and selective manner. Nevertheless, the way in which the basic ingredients are mixed and fed into the mold has not been modified, and still, basically includes the following steps: Two independent premixtures are prepared, the first one comprising a mixture of the activator and a fraction of the lactam, and the second one comprising the initiator and the remaining fraction of the lactam. Both premixtures are subsequently combined to carry out the polymerization. An example of this way of polymerization can be found in the U.S. Pat. No. 3,505,448.
The prior art techniques allow producing serial parts of one and same composition, but they are not suitable in the continuous manufacture of non-serial parts, for example with individual sizes and/or geometries, different characteristics or compositions.
Based always in this system of premixing the lactam with the initiator and the lactam with the activator, there are different systems for dosing premixtures. One possible technique is based on using gear pumps, as described for example in the U.S. Pat. No. 4,404,360-A. In U.S. Pat. No. 4,404,360-A a pair of tanks are used for the premixtures. An alternative technique is based on using a couple of extruders for making the premixtures and dosing them, as described in the United States patent application US 2012/0,313,290-A1. US 2012/0,313,290-A1 describes a process for the production of fiber-reinforced thermoplastic parts and an injection-molding machine. The injection-molding machine has a first extruder screw for liquefying and mixing a polymeric precursor and an activator and a second extruder screw for liquefying and mixing a polymeric precursor and a catalyst. In a chamber the contents, which have been previously liquefied in the extruder screws, are mixed, and then poured into a mold, where the polymerization takes place.
European patent application EP 2,338,665-A1 describes a process and device for polymerizing lactams in molds, in which the lactam, the activator and the initiator are independently fed and dosed into a mixing head that feeds a mold. The described process for polymerizing lactams can be catalyzed by a large group of initiators and activators, some of them being in solid state. Some initiators and activators must be melted and maintained at high temperatures during the whole process. The device described for molding lactams is versatile for a large group of initiators and activators. Said versatility makes the device complicated to use, since it requires heating means in each and every one of the dosing elements of the circuit through which each of these materials (initiator and activators) passes. The three components (lactam, activator and initiator) of the polymerization process are dosed separately. This leads to two of the advantages of the described process: (1) it ensures stability over time, and (2) it allows a different dosage (in %) of each component for each type of manufactured part.
However, it has been observed that melting and dosing the activator and the initiator at high temperatures is complicated. Furthermore, both materials must be added to the mixture in very specific percentages, which complicates its correct dosage using standard gear pumps. In addition, due to the fact that solid initiators and activators must be melted at temperatures higher than room temperature, and their molten state presents viscosities, which depend on temperature, their temperature must be maintained constantly and accurately along the entire dosing process.
The different elements of the device described in the patent document EP 2,338,665-A1 are each heated independently from the rest, by means of individual heating systems, either an electrical system based on resistances or by means of a heat transfer fluid. Nevertheless, the individual heating of these elements entails a cold area in the connecting element (by means of valves, bolts or whatever other connecting means) between elements. For example, the caprolactam becomes solid at temperatures below 70° C., and these cold spots may cause the subsequent cooling and solidification of the monomer, thus clogging the dosing systems.
Moreover, also the melting of the lactam in a pressurized tank at temperatures above 70° C. involves an extremely long time when there is a large amount of solid lactam to be melted (larger than 1 kg). In a continuous manufacturing process, it is preferable to melt the entirety of the lactam needed to manufacture the series of parts planned for a working day. In consequence, the amount of lactam to be melted is usually much larger than 1 kg (between 1 and 1000 kg), and the melting time is too long as a result. Furthermore, the lactam usually remains in a molten state for at least 8 hours, which increases the possibility of absorbing moisture and of possible thermal degradation. The electric consumption of maintaining the lactam tank at a temperature above 70° C. may also be excessive.
In sum, there is a need for a device that enables to solve in an efficient manner the aforementioned problems of conventional methods for polymerizing lactams in molds.